PROJECT SUMMARY/ABSTRACT Type 1 diabetes (T1D) management is challenging for young adults, who assume responsibility for self- management and transfer T1D care from the pediatric to adult healthcare system as they also manage typical developmental demands (e.g., social, financial, work/school, residential). Many young adults have extended (>6 months) gaps before following up in adult care, which increase their risk for poor health outcomes and serious health complications. There are few evidence-based programs to support young adults with T1D during the transfer period between pediatric and adult care. Novel, developmentally appropriate approaches are needed to help young adults negotiate the challenges of living with T1D and complete a timely transition to adult care. The goal of the proposed study is to evaluate a new intervention for young adults with T1D that maximizes diabetes-related social support. The proposed transition intervention builds on young adults' social support network and provides them access to relevant information about T1D care in adulthood. Around the time they leave pediatric T1D care, young adults with T1D will be paired with a trained ?Peer Mentor,? an experienced, slightly older young adult with T1D, who will meet with them in-person and have frequent (weekly, then monthly) contact for 12 months. During the contacts, the Peer Mentor will use strengths-based psychoeducational strategies to teach and model problem-solving and stress-management skills, encourage the young adult to seek support from their existing social support network (e.g., family, friends) and identify new sources of diabetes-related support, help the young adult prioritize T1D self-care during the very demanding young adult period, share his/her transition experiences and strategies for successfully navigating the adult healthcare system, and prepare for the first adult care visit. The Peer Mentor will be a positive role model and source of help, encouragement, and accountability around transition, with the goal of reducing isolation, promoting self-management, and preventing gaps in care. We will test this intervention with 150 young adults randomly assigned to the intervention or usual care. Our main goal is to evaluate the intervention's impact on important T1D outcomes, such as glycemic control, time to adult care, treatment adherence, and psychosocial well-being. Clinic appointment information, questionnaires, adherence data, and glycemic control biomarkers will be collected at baseline and 6 and 12 months later. The ultimate goal of this research is to validate a supportive intervention that can facilitate T1D self-management and transfer of care during the difficult young adult years and promote optimal T1D health outcomes.